The present invention relates to an image forming apparatus for fixing a developer image formed on a transfer medium with a fixing unit, thereby forming an image, and to a method of forming an image.
An image forming apparatus such as a copying machine or a printer has a fixing unit for heating and fusing a developer image formed on a transfer sheet, thereby fixing the developer image on the transfer sheet. This fixing unit generally has a heating roller incorporating a heat source such as a heater, and a press roller in rolling contact with the heating roller with a predetermined pressure. The transfer sheet formed with the developer image is conveyed through a portion between the heating roller and press roller. During conveyance, the developer image is fixed to the transfer sheet by the heat and pressure respectively applied by the heating roller and press roller.
In the fixing unit having the above arrangement, in order to sufficiently fix the developer image on the transfer sheet, the heating roller and press roller must be maintained at a predetermined temperature. For this purpose, usually, the surface temperature of the heating roller is detected by a temperature sensor such as a thermistor, and the heat source is controlled in accordance with the detection result.
According to a device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-179437, a reference temperature for a heating roller is set in advance in accordance with the size of a transfer sheet to be used, and the temperature of the heating roller is controlled on the basis of the reference temperature.
In a copying machine, various types of transfer sheets are prepared in accordance with the sizes of images to be formed, and transfer sheets having sizes corresponding to the images are selectively supplied. The widths, i.e., the axial lengths, of the heating roller and press roller of the fixing unit are set in accordance with the maximum-size transfer sheet to be used.
Therefore, in this fixing unit, when fixing is to be performed on a small-width sheet as a transfer sheet, a paper passing region where the transfer sheet passes and which comes into contact with the transfer sheet, and a paper non-passing region which does not come into contact with the transfer sheet, i.e., where the transfer sheet does not pass, are generated in a press contact portion between the heating roller and the press roller. When continuous copying operation is performed by using a plurality of these small-width sheets, the heat consumption amount differs between the paper passing region and the paper non-passing region, and the paper non-passing region is heated to a temperature higher than a predetermined temperature. In the subsequent copying operation, when a large-width transfer sheet is used, it partly passes the excessively heated paper non-passing region in the fixing unit. This leads to an image defect such as a high-temperature offset, or a paper passing defect such as a paper separation defect.
According to an apparatus disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 5-5112, when continuous copying operation is performed on a predetermined number of transfer sheets or more, the predetermined temperature of a heating roller is gradually decreased during the continuous copying operation, so that an excessive temperature increase of a paper non-passing region is prevented.
In recent years, a composite image forming apparatus, in which a plurality of functions such as those of a copying machine, a printer, and a facsimile are combined, has been proposed as an image forming apparatus. In this image forming apparatus, various types of image data are input from an external device such as a personal computer or from a communication line. These image data are subjected to mixing and processed by an interrupt to form an image. Accordingly, in the composite image forming apparatus of this type, small-width transfer sheets and large-width transfer sheets are mixedly used more often than in an analog copying machine, and an image defect and a paper passing defect in the fixing unit described above tend to occur.